Add constraints to limit dependency versions

[pavelzw]

Problem description

It would be cool if we were able to add constraints in pixi.toml as well without needing to install them

[dependencies]
numpy = "*"

[constraints]
abc = "<5"

In this scenario, if abc is added to the env, we get abc<5.

[bollwyvl]

Likely implied, but I would assume this would work for features as well.

I've presently been doing this via dedicated full dependency-only features that provide refinements e.g.

feature.run.dependencies.python = ">=3.9"
feature.run-oldest.dependencies.python = "3.9.*"
# the same for other factors

I guess an feature.oldest.constraints would allow all of those (even unrelated ones) to be stored together, and only impact the solve when needed.

Semi-related for the common case of minimum dependencies: another way to avoid extra specs that need to be kept in check fattening up [environments.*.features] with strategy:

[feature.py.dependencies]
python = ">=3.9"

[feature.test.dependencies]
pytest = ">=7"

[environments]
test = {features = ["py", "test"] }
test-oldest = {features = [
  {name = "py", strategy = "lowest"},
  {name = "test", strategy = "lowest-direct"},
]}

This would start to get crazy with solve-group, but it was already crazy.

As for export: I don't think environment.yml would be able to support this at all, as it doesn't expose an equivalent to run_constraints or run_constrained. pip would need a second file, though it could be specified inline with a second file e.g. -c env_platform_pip-constraints.txt.